The invention relates to a method for producing high-purity oxygen used for a medical treatment and the like, and a device for producing high-purity oxygen.
Conventionally, known methods for producing high-purity oxygen include: a method in which oxygen is separated from nitrogen in atmospheric air (hereinafter referred to as “air”) using PSA (Pressure Swing Adsorption); a method in which high-purity oxygen is obtained by cooling air using a very low-temperature cooler (cryocooler) and separating the liquid oxygen from other gases such as nitrogen after liquefying the oxygen through a difference between the liquefaction temperature of the oxygen and the liquefaction temperatures of the other gases in the air; and a method in which the oxygen is obtained through a combination of PSA and a cryocooler.
Among these methods, in the method using PSA, when a two-tower system is used, it is difficult to separate oxygen from argon, and therefore a purity of oxygen is limited to 90% to 96%. In order to obtain higher-purity oxygen, it is necessary to use an adsorbent for selectively adsorbing the oxygen to purify the oxygen as disclosed in, for example, Japanese Patent Publication (KOKAI) No. 2001-87616.
The liquefaction temperature of oxygen is −183.0° C. at the atmospheric pressure, whereas the liquefaction temperature of argon is −185.9° C. at the atmospheric pressure. Therefore, in the method using the cryocooler, since the difference in the liquefaction temperatures between the two gases is extremely small, it is very difficult to separate oxygen from argon. To solve this problem, Japanese Patent Publication (KOKAI) No. 05-203347, for example, has disclosed a method in which oxygen and argon are liquefied first, and then the mixture is fractionated to obtain high-purity oxygen.
Incidentally, the cryocoolers, particularly small cryocoolers, have been widely utilized for cooling a variety of devices for detecting a weak-signal as well as for a cryopump, a superconductivity application, and the like. The typical small cryocoolers currently available in the market include two types, namely Stirling cycle and Gifford McMahon cycle. The two types of cryocoolers use helium as a working gas, and generally achieve a temperature range of 150 K-4 K as a targeted temperature. In the Stirling cycle (strictly speaking, it should be called reverse Stirling cycle, but it is often called the Stirling cycle for the cryocooler), a compressor and an expander are used in a refrigeration cycle in which in principle a reverse Carnot cycle is conducted. The cryocoolers are known to be high performance and high efficiency.
Recently, pulse-tube cryocoolers have been developed and expected to replace the conventional cryocooler. The pulse-tube cryocooler does not require a cryogenic moving part (an expander), and is capable of operating by using helium gas. Further, the pulse-tube cryocooler can obtain the temperature range described above. In particular, it is known that the pulse-tube cryocooler with an inertance-tube system provides high refrigeration efficiency by generating variations in gas pressure using a compressor at a frequency near the resonance frequency of a vibration system composed of an inertance tube and a buffer tank (for example, see Japanese Patent Publication (KOKAI) No. 2001-304708).
Recently, there has been an increased demand for oxygen-production device as domiciliary medical equipment. In the domiciliary medical equipment, it is necessary to supply oxygen for an extended period of time when a user of the equipment is out of home. Therefore, it is desired to provide portable medical equipment in which liquefied oxygen is stored in a container.
When a gas with a boiling point lower than that of oxygen is contained at a high concentration, oxygen is easily evaporated at the beginning due to the higher boiling point, thereby obtaining a gas with a high oxygen concentration. However, as time elapses, the amount of the gas other than oxygen having a boiling point lower than that of oxygen increases, thereby reducing the oxygen concentration and causing a risk of oxygen deficiency. For this reason, in the equipment in which liquid oxygen is stored in a container and carried as described above, the law requires that the concentration of oxygen in the container shall be 99.5% or higher.
As described above, in recent years, the demand for the oxygen-production device as the domiciliary medical equipment has increased. More specifically, it has been desired that the oxygen-production device is capable of storing and carrying high-purity oxygen with an oxygen concentration of 99.5% or higher in the container.
In the conventional method of producing oxygen using PSA as disclosed in Japanese Patent Publication No. 2001-87616, it is difficult to obtain oxygen in the required liquid state. In order to obtain liquid oxygen, it is necessary to provide an additional oxygen-liquefying apparatus using the cryocooler, thereby making the system complicated.
In the conventional method of producing liquid oxygen using the conventional cryocooler, due to the extremely small difference between the liquefaction temperatures of oxygen and argon, it is difficult to separate the two gases, thereby making it difficult to obtain high purity liquid oxygen described above. In the method disclosed in Japanese Patent Publication No. 05-203347, it is possible to obtain high purity oxygen gas through the fractionation. However, it is not possible to directly obtain liquid oxygen, which has a boiling point higher than that of argon.
In view of the problems described above, the invention has been made, and an object of the invention is to provide a method and a device for producing liquid oxygen with an oxygen concentration of 99.5% or higher directly and with high efficiency in which oxygen in the air is effectively separated from nitrogen and argon.
Further objects and advantages of the invention will be apparent from the following description of the invention.